Copy machines are equipped with a developing device, which includes a toner density sensor for detecting a toner density in the device. When the toner density in the device becomes a predetermined value or lower, the image forming apparatus controls a toner replenishment motor so as to supply toner to the device so that a mixing ratio of the toner to carrier (T/C ratio) in the device can be maintained at a constant value.
Additionally, the image forming apparatus adjusts the image forming condition preventing the image density of the print image, as a toner image formed on the image carrier, from fluctuating because of variations in temperature or the like and time variations including a deterioration of the image carrier such as a photosensitive member or deterioration of toner.
The image forming apparatus defines a reference toner pattern for adjusting the image density between a plurality of print images as illustrated in FIG. 5B. Then, the image forming apparatus adjusts the image forming condition including a developing bias, an input and output data conversion table and the like based on the toner density of the reference toner pattern detected by the toner density sensor arranged so as to be opposed to the surface of the image carrier.
However, if the toner density in the developing device decreases because of lack of toner in a toner replenishment container for replenishing the toner to the developing device or other reasons, the amount of toner adhering to the reference toner pattern formed on the surface of the image carrier becomes lower than when the toner density in the developing device is not low and the same surface potential of the image carrier.
If the adjustment of the image forming condition is performed based on the reference toner pattern using an amount of toner with low adhesion, an excessive amount of toner may adhere to the image carrier when the toner density in the device is at a normal value after the adjustment by the toner replenishment motor, resulting in a high image density of the print image.
To resolve the problem, there is provided an image forming control method wherein, if the time to detect an adhering toner amount on the above-mentioned reference toner pattern (i.e., timing for performing the adjustment of the image forming condition) comes when the toner density in the developing device is at a predetermined level or lower, the timing is delayed by a predetermined period of time so that the image forming condition can be corrected.
However, if the image forming apparatus forms the reference toner pattern between images under printing so that the adjustment of the image forming condition is performed during the printing period so as to avoid a decrease in printing speed or other reason, the following issues may occur.
Specifically, in the prior art, the timing for detecting the adhering toner amount is delayed by the predetermined period of time in accordance with a decrease in toner density in the developing device. Therefore, the process is performed after the toner density in the developing device is actually decreased. For this reason, it is not possible to merely shift timing.